How to make preview for posts?

I wonder how can I make the post is like a forum. I mean, you cannot see the content if you don’t open the page/post. At the main page, usually it will show the contents of the posts but I don’t want it to be like that. If can I want to make it show a brief description of the post and if want to view the whole content must open the post’s page.

Hope you can understand. Thank you!